They sit on you like a weight, these horrible things you've done.
You don't ever escape them, really. No matter how many times over you've been forgiven, (pardoned, cleansed, justified, whatever) no matter how many years pass, you still feel them all. They change you in little ways. Maybe it's that you don't trust yourself like you used to: before, you clung to your own judgment, but leaned on his in tough situations. Now his takes the forefront more often. Maybe it's that you're just a little quicker to take blame, a little slower to judge, a little sooner to forgive: before, good things like these came easily on principle. Now they come from your gut, like it's purging itself, desperately still trying to erase all these things it knows you've done.
You'd never expect him to understand. How could he? How could anyone understand what it's like knowing forever and ever how miserably you screwed up? A whole chunk of your life was consumed with that one horrible, awful mistake and your constant desperate efforts to contain the results. You can never take that back. You can never have those years back. As for a Woman in White your guilt keeps you from the good old days, I can never go home. Your crimes will always be with you; a millstone around your neck.
There would have been a time this knowledge, certainty, that you're alone in your struggle would have scared you, made you broody, made you want to express yourself to him. Maybe it's that gut thing again, but your isolation doesn't affect you like that now. You don't want anyone to ever understand. You want just the opposite. You want with a burning, passionate, fiery, desperate need – with every sympathetic bone in your body – to SAVE THEM from ever having to know or understand.
That's why it terrifies you when you realize he's a demon. The world seems to warp around you because no. Cause he took the steps to get him here. Sure he saw it and tried to stop it. But your brother will see only the road of guilt when he gets to the other side (oh, he WILL get to the other side) and you fear for him. Already, you're plotting your excuses, planning your forgiveness, to ease the process, chip away at the weight. "It's not your fault, Dean. You didn't know. You didn't want this. It's Crowley's fault, it's Cain's - what you did. It wasn't you, it was them. They got you into this. It's not your fault." It's a constant litany in your head, a strengthening exercise for you – you, the one who will have to be ready to hold Dean's weight – like packing the First Aid kit so you're ready to patch and dose him up, so you're ready for the pain you know is in store for him.
You never wanted him to understand; yours was meant to be a load not shared. Now all you have is hope that he'll let you help him carry the burden.